The present invention relates to a shifting mechanism of the "X-Y" type for use with a shift bar housing assembly of a mechanical change-gear transmission, and more particularly, to such a mechanism of the type which is automatically or semi-automatically shifted, on a transmission which otherwise would be manually shifted.
Shift bar housing assemblies for mechanical change-gear transmissions usually comprise a plurality of generally parallel, independently transversely movable shift bars, which are to be selected, and moved transversely, to effect engagement/disengagement of a particular gear ratio. Such shift-bar housing assemblies are well known to those skilled in the art, as may be seen by reference U.S. Pat. Nos. 2,951,392; 4,567,785; and 4,873,881, all of which are assigned to the assignee of the present invention and incorporated herein by reference.
Typically, such shift bar housings have been manually controlled, and operated by a shift finger fixed to a directly mounted shift lever, or to the cross-shaft of a remotely controlled shifting mechanism. Automatic and semi-automatic mechanical transmissions, based upon generally standard mechanical transmissions, but provided with electric or pneumatic actuators, are now well known in the prior art, as may be seen by reference to U.S. Pat. Nos. 4,361,060 and 4,648,290, the latter of which is assigned to the assignee of the present invention, and both are incorporated herein by reference.
Also now well known in the prior art are automatically controlled "X-Y" type shifting mechanisms, wherein a shift finger is automatically moved in the axial (X-X) or rail selection direction, and then in the transverse (Y-Y) or gear engagement/disengagement direction. It will be understood by those skilled in the art that reference herein to the "transverse" and "axial" directions pertains to the shifter, and do not necessarily imply any particular direction on either the transmission or the vehicle. Also known are single shaft shifting mechanisms, as may be seen by reference to U.S. Pat. No. 4,920,815, which is assigned to the assignee of the present invention, and the disclosure of which is incorporated herein by reference.
Frequently, such an automatic or semi-automatic X-Y shifting mechanism is applied to a non-synchronous mechanical change-gear transmission. As used herein, the term "non-synchronous" will be understood to refer to a mechanical change-gear transmission of the type not equipped with synchronizers (or with less than extremely accurate synchronizers). On such a transmission, one of the problems associated with using an automatic or semi-automatic shifting mechanism is the occurrence of non-synchronous clutch engagements, i.e., an engagement wherein the mating jaw clutches come into contact with each other (typically, on the transverse, end surfaces of the jaws), but don't actually engage in the manner intended for such clutches.
Typically, one result of a non-synchronous clutch engagement is a phenomenon known as "kick-out", in which the jaw clutch being moved into engagement with another jaw clutch is only partially engaged, and then is effectively kicked out of engagement. Such kick-out movement is normally transmitted from the jaw clutch, back through the shift fork, the shift finger, and the shifting mechanism. This kick-out force being transmitted back through the shifting mechanism can cause damage to any one of a number of components of the shift system. By way of example only, in electrically actuated shifting mechanisms of the type illustrated in above-incorporated U.S. Pat. No. 4,873,881, a rotary input from an electric motor provides an input to the shifting mechanism by means of a recirculating ball-screw device. When such a shifting mechanism is subjected to a kick-out force, even if most of the system components can withstand the force without breaking, one typical result is damage to the threads in the ball-screw mechanism. As will be understood by those skilled in the art, thread damage in such a mechanism can render the mechanism nearly inoperable.